Azobenzene Liquid-crystalline Polymer Research Articles

Preparation and Photoresponsive Behavior of Plastic Films Coated with Azobenzene Liquid-Crystalline Polymer Layers

Plastic films coated with crosslinked liquid-crystalline polymer (CLCP) layers containing azobenzene moieties (plastic/CLCP bilayer films) were prepared by thermal polymerization of mixtures of an azobenzene monomer and a crosslinker placed between a plastic film and a glass substrate. When the glass substrate was untreated, the obtained bilayer film bent slightly away from the UV light source when irradiated on the CLCP layer side. This bending direction was the same as that of a hybridly aligned CLCP film. When the glass substrate was coated with a rubbed polyimide to strengthen the homogeneous alignment of the azobenzene moieties of the CLCP surface layer, the obtained bilayer film bent toward the irradiation direction of the incident UV light. It was the same as a homogeneously aligned CLCP film and the degree of bending was larger than that of the untreated sample. This means that the photoinduced bending behavior of the plastic/CLCP bilayer films strongly depends on the alignment of the azobenzene moieties of the CLCP surface layer facing the incident light.

Surface patterns induced by laser irradiation on thin polymer bilayer films

We report on the preparation of wavelike surface patterns with characteristic wavelengths on thin bilayers of poly(methyl methacrylate) on azobenzene liquid crystalline polymer films (LCP/PMMA) by irradiation of a single polarized pulsed laser beam. The formation of such patterns was influenced by the thickness of the upper layer and the laser fluence. We were also able to guide the wavelike pattern to have a specific orientation by placing an elastic polydimethylsiloxane (PDMS) mold on the surface of bilayer film prior to laser irradiation. Moreover, the property of the laser irradiation, that is, the selectivity through mask-projection systems, allowed us fabricating complicated micropatterns for novel microdevices.

Photoinduced chirality in achiral liquid crystalline azobenzene polymers containing different polar side groups

Circular dichroism (CD) was induced in the films of two achiral liquid crystalline polymers, poly[(4′-(6-(methacryloyloxy)hexyl)oxy)-(4-X)azobenzene] (PM6X, X = C and N for cyano and nitro, respectively), by irradiation with one handed circularly polarized light (CPL) at 442 nm. The CD and UV spectra of the polymeric films suggest that left-CPL induces a left helical structure and right-CPL reverses the resulting structure into a right helix. For PM6C, the photoinduced CD values of the nematic film are much higher than in the casting and isotropic film. The photoinduced chirality of PM6C films is higher than PM6N due to a higher polarity of nitro groups in PM6N. The PM6N’s CD intensity around 450 nm is increased with the irradiation time of linearly polarized light (LPL) followed by left-CPL irradiation. The correlation between azobenzene chromophores and circularly polarized light will be discussed in the paper.

Surface Relief Grating Couplers Using Azobenzene Liquid-Crystalline Polymer Films

Input and output grating couplers with a grating period of 1.3 µm were fabricated in an azobenzene liquid-crystalline polymer film by holography. When a He-Ne laser beam at 633 nm was incident to the input grating, the beam was coupled, propagated in the polymer film, and then was taken out from the output grating. The coupling efficiency of the input and output coupler was estimated as about 1.1% in TE mode. Upon irradiation of the film with UV light to cause trans-cis photoisomerization of the azobenzene moiety, the intensity of the output beam was repeatedly changed.

Effects of Intensity of Actinic Light and Temperature on Photochemical Phase Transition of Azobenzene Liquid Crystals Probed by a Near-Infrared Laser Beam

ABSTRACT The change in transmittance of a 1.55-μm laser beam was investigated by a photochemical phase transition of an azobenzene liquid-crystalline polymer. Effects of the intensity of actinic light and temperature on the photoresponsive behavior were explored using laser beams at 633 nm and 1.55 μm. It was found that the intensity of the actinic light or temperature affected on the response, regardless of the wavelength of the probe beam.

Synthesis and Properties of Highly Birefringent Azo-Tolane Liquid-Crystalline Polymers: Effect of the Position of the Tolane Moiety in the Side Chain

ABSTRACT We have synthesized two types of azobenzene liquid-crystalline polymers with a tolane moiety exhibiting high birefringence: one is the copolymer having an azobenzene moiety and a 3-ring tolane group in the side chain (PA-3T) and the other is the polymer containing a long mesogen core, in which the azobenzene group is directly connected to the tolane moiety (P3AT). The values of 0.35 and 0.39 in birefringence were obtained in PA-3T and P3AT, respectively. When the film was irradiated with a writing beam at 488 nm, the change in birefringence was efficiently induced with P3AT compared with PA-3T.

Spontaneous motion observed in highly sensitive surface relief formation system

Surface relief gratings formed on a liquid-crystalline azobenzene polymer film known as a highly sensitive system were investigated to obtain deeper insights into polymer migration on the micrometer scale. A surface relief formation was induced with incoherent nonpolarized patterned blue-light using a photomask instead of interference patterns of a coherent laser. Spontaneous lateral migration was observed under the edge of the grating photomask at room temperature even after terminating light irradiation. It was found that interfacial tension and the living cis form of azobenzene are essential for inducing spontaneous motion.

Highly Efficient Photogeneration of Surface Relief Structure and Its Immobilization in Cross-Linkable Liquid Crystalline Azobenzene Polymers

A new family of azobenzene containing polymer applicable for photoinduced surface relief formation is reported. An azobenzene containing acrylate was copolymerized with methacrylate monomers bearing an oligo(ethylene oxide) chain via free radical copolymerization, yielding soft liquid crystalline polymers. These copolymers displayed a smectic liquid crystal phase at room temperature. After preexposure to ultraviolet light, thin films of the liquid crystalline polymers showed highly sensitive photoinduced material transfer to generate the surface relief structures. The typical exposure dose required for full polymer migration was as low as 50 mJ cm-2. The inscribed surface relief structures could be rapidly and fully erased either upon incoherent nonpolarized ultraviolet light irradiation or by heating close to the clear point of the soft liquid crystalline polymers. On the other hand, the chemical cross-linking, using a mixed vapor of hydrogen chloride and formaldehyde after surface relief inscription, re...

Unconventional polarization characteristic of rapid photoinduced material motion in liquid crystalline azobenzene polymer films

We carried out double-beam interference experiments using an argon ion laser with controlled polarization in order to cause the formation of photoinduced surface relief in liquid crystalline azobenzene polymer films. The irradiation was undertaken on a film in the cis-rich state obtained by a pre-exposure to ultraviolet light. In this procedure, the efficiency of the photoinduced mass transfer was high, more than 1000 times greater than hitherto reported for amorphous azobenzene polymers. This approach revealed the unusual nature of the migration process. Rapid mass migration is promoted by intensity holographic recording, independent of the polarization of the light used for the irradiation. This insensitivity with respect to the polarization of the light led us to the conclusion that rapid mass migration starting from a cis-rich azobenzene polymer is predominately driven by phototriggered elemental processes such as local dewetting, self-organizing motion, and translation diffusion.

Dynamic Temperature Distribution in Photo‐induced Alignment of Azobenzene Liquid Crystalline Polymer

AbstractA theoretical model has been established to describe the dynamic temperature distribution during the alignment of an azobenzene liquid crystalline polymer irradiated by a linearly polarized laser beam. The dynamic heat diffusion equations are used, and the relationship between heat source item and time is introduced, based on experimental results. With the model, the contours of the temperature distribution at different time have been worked out. It can be found from the theoretical model that there is a maximum temperature rise during the photo‐induced alignment, which is coincident with the analysis of experimental observations. The existence of a minimum laser power and an offset photo‐alignment temperature Toff required to carry on photo‐induce alignment are explained based on the theoretical model.Temperature distribution after two minutes.imageTemperature distribution after two minutes.

Instantaneous temperature field induced by laser pulse in azobenzene liquid crystalline side chain polymers before alignment

Dynamic results have been worked out according to the existing theoretical model, to describe the temperature field that is established within a film of isotropic trans azobenzene liquid crystalline side chain polymers when a linearly polarized laser beam illuminates it. The pulse times adopted in calculations are 10 ns, 100 ns, 0.01 ms, 0.1 ms and 1 ms. The results with a pulse time of 1 ms, after which a temperature field is found to be formed, are discussed in detail. It has been found that (1) the rate of change in temperature will be larger when heat conductivity is smaller; (2) the temperature increases as the radius of the laser increases; (3) for a pulse time of 1 ms, the maximum temperature rise is 10.5°C and after 0.9 ms the temperature field disappears.

Azobenzene-Containing Thermoplastic Elastomers: Coupling Mechanical and Optical Effects

Thermoplastic elastomers of a styrene−butadiene−styrene (SBS) triblock copolymer containing grafts of an azobenzene side-chain liquid crystalline polymer were prepared by polymerization of an azobenzene acrylate monomer in solutions with dissolved SBS. Stretching thin films at room temperature induces long-range orientation of the azobenzene mesogenic groups along the strain direction. Upon irradiation with polarized or unpolarized UV light at 360 nm, the orientation of azobenzene moieties in stretched films is erased as a result of the trans-to-cis photoisomerization, while on subsequent irradiation with polarized or unpolarized visible light at 440 nm, the initial stretching-induced orientation is recovered due to the cis-to-trans back-isomerization. Gratings can be inscribed on stretched films at different alignments with respect to the strain direction, whose fringe spacing can be modified through elastic extension or retraction of the films.

Control of Chirality of an Azobenzene Liquid Crystalline Polymer with Circularly Polarized Light

Irradiation with circularly polarized light of a film of an achiral azobenzene liquid crystalline polymer induces chirality. Circularly polarized light with opposite handedness produces enantiomeric structures. A chirooptical switch based on this phenomenon is reported.

Optically controlled image storage in azobenzene liquid-crystalline polymer films

We report on optically controlled image storage based on photo-induced alignment in azobenzene liquid-crystalline polymer films. Measurements reveal that the polymer film possesses photo-induced birefringence of large magnitude and the characteristic of long-term optical storage. The photo-induced alignment process is used for storing images. The stored image is read from the film placed between two crossed polarizers. It is demonstrated clearly that the alignment of the azo chromophores as well as the stored image can be easily controlled by choosing the appropriate polarization of the irradiating light or by rotating the film.

Photochemistry Determined by Light Propagation. Part 1 Three-dimensional Photomanipulation of Self-organized Azobenzenes in Liquid-Crystalline Polymers

Abstract Clear-cut observation of the reorientation and self-organization of azobenzene chromophores to the propagation direction of actinic light is described, using spin-cast thin films of a liquid crystalline azobenzene polymer.

Surface plasmon resonance research on photoinduced switch properties of liquid crystalline azobenzene polymer Langmuir–Blodgett films

We report a novel phenomenon of photoisomerization-induced switch properties in liquid crystalline azo-benzene polymer poly{2-hydroetheyl methacrylate}-co-{6-[4-(S-2-methyl-1-butyloxycarbonylphenylazo) phenoxy]hexyl methacrylate} Langmuir–Blodgett (LB) monolayers. These monolayers were deposited on gold film and irradiated with UV (360 nm) and blue (450 nm) light. The switch properties were investigated by the surface plasmon resonance (SPR) technique. A method called photoinduced patterning was used for the fabrication of a model molecular device. It has been found that the photoinduced process of writing and erasing in such devices can be repeated for tens of cycles.

Photoinduced properties of liquid crystalline azobenzene polymer in Langmuir–Blodgett films investigated by surface plasmon resonance

A novel phenomenon of photoisomerization-induced switch properties in liquid crystalline azobenzene polymer poly{2-hydroethylmethacrylate}-co-{{6-[4-( s-2-methyl-1-bufyloxycarbonylphenylazo)phenoxy]hexylmethacrylate}} Langmuir–Blodgett (LB) monolayers is reported. These monolayers were deposited on gold film and irradiated with UV (360 nm) and blue (450 nm) light. The switching properties were investigated by the surface plasmon resonance (SPR) technique. It has been shown that it is possible to switch the refractive index reversibly by irradiating these films with light of appropriate wavelength. Evidence shows that the surface pressure has an influence on the evolution of the resonance angle.

Surface Plasmon Resonance Investigations of Photoinduced Changes in Liquid Crystalline Azobenzene Polymer Langmuir-Blodgette Films

In this letter, we report a novel phenomenon of photoisomerization-induced changes in liquid crystalline azobenzene polymers P12 Langmuir-Blodgette monolayer deposited on gold film by irradiating the monolayer with UV (360 nm) and blue (450 nm) lights, which was investigated by surface plasmon resonance technique. It has been shown that it is possible to switch the refractive indices reversibly by irradiating these films with light of appropriate wavelength.